Retransmission control by a conventional OFDM communication apparatus using an interleave technology will be explained with reference to FIG. 1. FIG. 1 is a block diagram showing a conventional OFDM communication apparatus using an interleave technology. Retransmission control by the conventional OFDM communication apparatus using an interleave technology will be explained below, taking the following case where a first communication apparatus and a second communication apparatus both equipped with the OFDM communication apparatus shown in FIG. 1 perform a radio communication as an example. What will be explained here is a case where the first communication apparatus sends a signal to the second communication apparatus and when the signal received by the second communication apparatus contains an error, the first communication apparatus retransmits (resends) this erroneous signal to the second communication apparatus.
First, in the transmission system of the first communication apparatus, a transmission signal is stored in retransmission control section 11. This transmission signal is a packet-unit signal. The stored transmission signal is sent by retransmission control section 11 to interleave processing section 12 according to a transmission timing.
In interleave processing section 12, the sequence of the signal sent by retransmission control section 11 is rearranged according to a specific rule. The signal with the rearranged sequence is subjected to predetermined transmission OFDM processing by transmission OFDM section 13 and placed in each subcarrier.
Here, the signal subjected to the predetermined transmission OFDM processing above refers to a signal assigned to each subcarrier at predetermined subcarrier intervals as a result of interleave processing by interleave processing section 12. That is, regarding the signal subjected to the predetermined transmission OFDM processing above, the first to third transmission signals input to interleave processing section 12 are placed at intervals of, for example, 4 subcarriers such as subcarrier 1, subcarrier 5 and subcarrier 9, and so on.
The signal subjected to the transmission OFDM processing is sent to the second communication apparatus via antenna 14. The signal sent from the first communication apparatus is received by the second communication apparatus through a transmission path.
The signal received through antenna 14 by the second communication apparatus is subjected to predetermined reception OFDM processing by reception OFDM section 15. The signal subjected to the predetermined reception OFDM processing above is subjected to deinterleave processing by deinterleave processing section 16. The signal subjected to deinterleave processing is subjected to error correction processing by error correction section 17. The error-corrected signal is output to retransmission control section 11.
When the error-corrected signal in retransmission control section 11 contains no error, this signal is output as a reception signal. On the contrary, when the error-corrected signal contains some error, this signal is stored in predetermined memory. Then, a signal including a packet requesting retransmission of this signal is processed by interleave processing section 12 and transmission OFDM section 13 and then sent to the first communication apparatus through antenna 14.
Then, in the first communication apparatus, retransmission control section 11 sends the packet requesting retransmission by the second communication apparatus to interleave processing section 12 according to a retransmission timing. This packet is subjected to the same processing as that described above and resent to the second communication apparatus through antenna 14.
As shown above, the signal containing an error detected by the second communication apparatus is resent by the first communication apparatus.
However, the conventional OFDM communication apparatus using an interleave technology has the following problems. That is, there are cases where signals of poor quality concentrated on a specific time period are input as the signals subjected to error correction processing in the second communication apparatus.
Here, FIG. 2 is used as a reference to explain this situation more specifically. FIG. 2 is a schematic diagram showing an example of placement of subcarriers of a signal received by the conventional OFDM communication apparatus using an interleave technology. Suppose interleave processing section 12 in the first communication apparatus performs the interleave processing as shown in the example above.
When the signal with subcarriers placed as shown in FIG. 2 is received by the second communication apparatus, the signals output from deinterleave processing section 16 are signals picked up in time series from each subcarrier at intervals of 4 subcarriers such as subcarrier 1, subcarrier 5, subcarrier 9 and subcarrier 13, . . . . Here, as is apparent from FIG. 2, signals placed in subcarrier 1, subcarrier 5, subcarrier 9 and subcarrier 13, . . . are of poor quality.
As a result, signals input to error correction section 17 are signals of poor quality concentrated on a specific time period, which causes the effect of error correction by error correction section 17 to reduce, making signals with errors often output to retransmission control section 11. This causes the first communication apparatus to resend the same packet.
Furthermore, when a variation of the channel (transmission path) state occurs slower than the time interval at which the first communication apparatus sends the same packet, the channel state when the same packet above is sent for the first time becomes virtually the same as the channel state when the above same packet is retransmitted (resent).
In this case, when the signal including the resent packet is received by the second communication apparatus, the state of subcarrier placement in this received signal has virtually the same state as that shown in FIG. 2. Therefore, there is an extremely high probability that the packet resent from the first communication apparatus will also have errors in the second communication apparatus, and furthermore errors will occur consecutively in the above packets. This means that it will take a long time for the second communication apparatus to receive a specific packet sent by the first communication apparatus without errors.